1. Field of the Invention
The present invention relates to a novel alkylsulfonium salt and a novel photoresist composition containing the same or other alkylsulfonium salt, and in particular to a novel alkylsulfonium salt in which deep ultraviolet (hereinafter "U.V."); light having a wave-length of 220 nm or less is absorbed and which efficiently generates photoacids (i.e. protonic acids), and in addition to a novel photoresist composition which contains the same or other alkylsulfonium salt and is suitable for exposure to deep U.V. light having a wavelength of 220 nm or less.
2. Disclosure of the Related Art
Recently, in the field of semiconductor devices, integrated circuits and the other various electronic devices in which fine processing has been required, photoresists have been extensively used, and highly densified and integrated devices have increasingly been desired. Thus, requirements for photolithography technology, which is used to achieve fine patterning, have become increasingly strict.
Such fine patterning has been performed by using a light exposure having shorter wavelengths to pattern of the photoresist. In general, resolution (line width) R in an optical system is defined in terms of Rayleigh's equation: EQU R=k.multidot..lambda./NA
wherein .lambda. represents a wavelength of a light source for exposure, NA represents numerical aperture of the lens and k represents a process factor. It is seen from this equation that higher resolution, i.e. a small R value, is attained by shortening the wavelength .lambda. of the exposure light in a photolithography. For instance, in manufacturing a dynamic random access memory (hereinafter referred to as "DRAM") having a level of integration up to 64M, resolution of the minimum pattern size 0.35 .mu.m line-and-space has been required and for this reason, a g-line (438 nm) or i-line (365 nm) of a high-pressure mercury vapor lamp has been used as a light source to date. However, in manufacturing a DRAM having an integration level of of 256M or more, in which even finer processing techniques (processing size of 0.25 .mu.m or less) are required, it is believed that light having shorter wavelengths, such as deep U.V. light, and excimer laser beams (KrF: 248 nm, KrCl: 222 nm, ArF: 193 nm, F.sub.2 : 157 nm) can be effectively used, as taught in T. Ueno et al., Short Wavelength Photoresist Materials-Fine Processing for ULSI, Publisher Bunshin, 1988. In particularly, KrF lithography is presently being actively investigated.
With regard to photoresists, high integration has been investigated on the basis of multilayer (two or three layers) resist processes in place of conventional single-layer resist processes. As for the two-layer resists, there are known, for instance, resists (two-layer resists having silylated novolak resin as the upper layer) described in Wilkins et al., Journal of Vacuum Science and Technology, B3, 306-309, 1985.
With regard to resist materials for use in fine processing, requirements for high sensitivity to exposure light increase in addition to high resolution corresponding to reduction in the processing size. This is based on the fact that it is necessary to realize improvement in the laser's cost performance because the gas life of the excimer laser light source is short and the laser itself is expensive. In order to attain high sensitivity of the resist to light, chemically amplified type resists, in which a photoacid generator is utilized as a photosensitizer, have been developed and investigated in detail as resists for use in the KrF excimer laser, as described in H. Ito and C. Grant Willson, American Chemical Society Symposium Series, Vol. 242, pp. 11-23, 1984. A photoacid generator means a material for generating an acid by light irradiation. In the chemically amplified type resist containing the photoacid generator, a protonic acid generated by the photoacid generator is moved to the solid phase of the resist in the course of a post-exposure baking treatment and thus, amplifies catalytically chemical change in the resist material several hundred times to several thousand times. This resist attains remarkably high sensitivity as compared with conventional resist having photoreaction efficiency below 1 (reaction efficiency per one photon). As for the photoacid generator for use in chemically amplified type resist, there were known, for instance, triphenylsulfonium salt derivatives described in J. V. Crivello et al., Journal of the Organic Chemistry, Vol.43, No.15, pp.3055-3058, 1978; 2,6-dinitrobenzyl esters described in T. X. Neenan et al., Proceedings of SPIE, Vol.1086, pp.2-10, 1989; and 1,2,3-tri(methanesulfonyloxy)benzene described in T. Ueno et al., Proceedings of PME'89, Kohdansha Co., pp.413-424, 1990.
Most of the resists under development at the present time are such chemically amplified type resists. The development of high photosensitive materials corresponding to exposure sources with shortened wavelengths is essentially performed by using the chemical amplification mechanism.
A chemically amplified type resist for exposure to light from the KrF excimer laser needs transmittance of 60% or more per 1 .mu.m in thickness. In such a resist, the transmittance at the exposure wavelength is important to resolve the pattern.
However, even if the chemically amplified type single-layer resist for exposure to the g-line, i-line or KrF excimer laser beams, which is broadly used at the present time, is exposed to the light having wavelengths shorter than 220 nm (for instance, to the ArF excimer laser beams (193 nm)), generally the pattern cannot be resolved because of very strong absorption of the light to the resist. Namely, in the single-layer resist having a thickness of about 0.7-1.0 .mu.m, the exposure light is mostly absorbed to the resist in the vicinity below its surface at the incident side of the light and, thus the light almost does not reach a portion of the resist near the substrate. As a result, the portion of the resist near the substrate is almost not exposed to light and thus the patterns are not resolved. For this reason, in photolithography in which an ArF excimer laser is the light source of the next generation following the KrF excimer laser, existing resists are not exposed to light and thus patterns are not quite resolved. The conventional photoacid generators, including Crivello et al.'s triphenylsulfonium salt derivatives which are contained in the chemically amplified type resist as mentioned above, greatly absorb exposure light having wavelengths of 220 nm or less, because all these compounds have an aromatic ring in their structure. For the above-mentioned reason, existing photoacid generators cannot be applied to chemically amplified type resists which are exposed to exposure light having wavelengths of 220 nm or less, with which higher resolution of the pattern can be expected.
With regard to a polymer to be used as the base of the resist, there are the same problems as in the photoacid generator. The polymers such as novolak resin which is used in most of the existing resists for i-line and poly(p-vinylphenol) and which is broadly used at the present time as a basic polymer of the chemically amplified type resist for exposure to the KrF excimer laser beam, have an aromatic ring in their molecular structure. This is based on the fact that it is necessary to include a number of strong unsaturated bonds in the molecular structure of the resist in order to attain sufficient resistance of the resist to a dry etching process, following the patterning process, in a method of fabricating the semiconductor device. Thus, the aromatic ring was included in the polymer for the resist as necessary and as an indispensable structure for sufficiently attaining the intended object. As mentioned above, the requirements for fine processing have become increasingly strict and further reduction in size of the pattern has been investigated. As for the resist using the KrF excimer laser as the light source having shorter wavelengths than in the i-line, poly(p-vinylphenol) has been broadly used in place of novolak resin having strong absorption at 248 nm. This resin is transparent to the KrF excimer laser beam (248 nm)(transmittance is on the order of about 70% when the film thickness is 1 .mu.m), but has strong absorption at the wavelength region shorter than 248 nm because of the aromatic ring included in its structure. Thus, the resin cannot be utilized as the resist for lithography in which light of shorter wavelengths than in KrF, in particular light having wavelengths of 220 nm or less, is used as the exposure light. As for resin which is transparent at the wavelength region of 220 nm or less, there is a methacrylic resin, for instance, poly(methyl methacrylate) or the like. Although polymers which do not have an aromatic ring in their molecular structure exhibit transparency to light of 220 nm or less, such polymers exhibit no resistance to the above dry etching process and, as a result, cannot be utilized as the photoresist. In order to solve this problem, there is provided, resist comprising an allcyclic polymer, as reported in Takechi et al., Journal of Photopolymer Science and Technology, Vol.5, No.3, pp.439-446, 1992. In the report, a copolymer of poly(adamantyl methacrylate) and poly(tert-butyl methacrylate) has been proposed as a polymer having transparency to light at 193 nm and dry etching resistance.
As mentioned above, polymers for use in lithography which is carried out at wavelengths of 220 nm or less were reported in a few publications, but the photoacid generator which can be combined with these polymers and is necessary and indispensable to develop a chemical amplification action essential for improving the in cost performance of the laser has not been reported in any publication.